There are three main types of hearing loss:
Conductive hearing loss is present when the outer ear canal to the eardrum and the tiny bones (ossicles) of the middle ear cannot conduct sounds efficiently so that sound is not reaching the inner ear, the cochlea.
Sensorineural hearing loss is present when the inner ear (cochlea) or the nerve that transmits the impulses from the cochlea to the hearing centre in the brain or in the brain is damaged. The most common reason for sensorineural hearing loss is damage to the hair cells in the cochlea.
Mixed hearing loss is a combination of the two types of hearing loss discussed above.
Typically, a hearing impaired human suffering from sensorineural hearing loss experiences a loss of hearing sensitivity that is 1) frequency dependent and 2) dependent upon the loudness of sound at an ear.
Thus, a hearing impaired human may be able to hear certain frequencies, e.g., low frequencies, as well as a human with normal hearing, while other frequencies are not heard as well. Typically, hearing impaired humans experience loss of hearing sensitivity at high frequencies.
At frequencies with reduced sensitivity, the hearing impaired human is often able to hear loud sounds as well as the human with normal hearing, but unable to hear soft sounds with the same sensitivity as the human with normal hearing. Thus, the hearing impaired human suffers from a loss of dynamic range.